In numerous technologies webs of flexible material such as paper or plastics material are used as raw materials and are caused to travel at high speed, for example while unwinding them from a reel. It is important to maintain a constant tension of the web during its travel and to distribute this tension as uniformly as possible over the width of the web whatever the anomalies which may occur, such as for example an elongation of one side with respect to the other or waving of the edges.
This is the case, for example, in the manufacture of corrugated cardboard, where unequal tension over the length and/or width of one of the sheets of paper used in manufacture may result in faults in the corrugated cardboard produced.
Generally, one acts on the tension of the web, travelling under the action of pulling means generally constituted by a machine ensuring the transformation of this web, such as for example a machine for connecting this web to other webs, by modulating a braking action applied to this web upstream of the drive means taking into account its direction of travel. To this end a roller is provided in the path of the web, in contact with one side of the latter, which roller is mounted to rotate freely about a transverse axis with respect to the direction of travel and which is able to move over a short distance in a direction perpendicular to the web, depending on the pressure which the latter applies thereto. The movement of the axis of the roller controls a brake acting downstream of the direction of travel of the web, as for example when it is being unwound from a reel, to establish regulation of the tension of the web resulting in a tendency of the roller to remain in a predetermined mean position, depending on the predetermined longitudinal tension to be maintained in the web. To ensure that this tension is distributed as uniformly as possible over the width of the web, a second roller is also provided in the path of the latter, downstream of the roller controlling the braking, which second roller is also in contact with one of the sides of the web and is able to rotate freely about its axis which is arranged perpendicular to the direction of travel. One of the ends of this roller is pivoted at one side of the travelling web on a fixed support and its other end, located on the other side of the web, is able to move at right angles to the latter depending on any possible transverse deformation of the web resulting in different longitudinal tensions over its width, in order to tend to distribute uniformly over the width of the web, the longitudinal tension regulated by the play of the first roller.
One drawback of this system resides in the necessity of making the web follow a complicated circuit since, in order to be effective, the two successive rollers must deflect the latter. This drawback is particularly noticeable if one takes into account the high speeds, of the order of several hundreds of meters per minute, at which modern machines process travelling webs.
Furthermore, installations for carrying out the system are expensive, since it is necessary to provide not only the two above-mentioned rollers, but also the associated regulating means.
Finally, the efficiency of such an installation is very limited.
In fact, the traction exerted by the web on the first roller may cause a complete vertical movement of the latter without this resulting in a complete correction of the increase in tension of the web which is the cause of this, and consequently it may happen that, at certain points of the web, tensions occur which may cause rupture of the latter. As regards the movement of the second roller, this is generally left to the judgement of an operator, depending on any transverse deformation of the web which he observes, which renders an instantaneous correction of the fault impossible, the high speed of travel of the web being incompatible with a correction of this fault by a manual action upon the fault being sighted.